bug#41645: 27.0.91; Combining Grapheme Joiner (#x34f) gui artifacts

[David Fussner via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

On Tue, 2 Jun 2020 at 15:35, Pip Cet <pipcet@gmail.com> wrote:
>
> David Fussner <dfussner@googlemail.com> writes:
> > A couple of data points, in case they're helpful:
>
> Thanks again for testing.
>
> > On 27.0.91 _unpatched_, I see the artifact whenever the font of the
> > CGJ is different from that of the glyph before it, no matter which
> > script I'm using. When the font of the CGJ and the previous glyph are
> > the same, I don't see the artifact, except in Hebrew, where it's still
> > present.
>
> Which font are you using for Hebrew text?

DejaVu Sans, in this instance, which at least has its own CGJ.

>
> > C-u C-x = displays the CGJ on its own, as a separate glyph,
> > whenever it's used in Hebrew and also whenever its font doesn't match
> > that of the glyph before it. When the font does match, in Latin or
> > Greek script, the cursor doesn't stop on the CGJ, and C-u C-x = shows
> > it as composed with the previous character.
>
>
> That sounds as it should be. I'm not sure I understand what you're
> seeing in Hebrew text, though: you said you saw the artifact there, but
> also that the CGJ is displayed as a separate glyph. Is that corrcet?

Yes, C-u C-x = doesn't present the CGJ as having been composed with
anything else in Hebrew text.

>
> > With Pip Cet's second patch, 27.0.91 shows exactly the same behavior
> > with C-u C-x =, but the visual artifact never appears, at least in my
> > testing, neither in Hebrew nor in the LTR scripts.
>
> So that sounds like an improvement.
>
> While I think we definitely want the patch I sent , it doesn't solve the
> real issue: zero-width glyph strings. If we want to allow those, a lot
> of the display code has to be changed (we're going to have to figure out
> how to show the cursor, for starters); if we don't, that's a change to
> the composition-function interface, albeit a minor one.
>
> > Hope this helps.
> >
> > On Mon, 1 Jun 2020 at 23:37, Pip Cet <pipcet@gmail.com> wrote:
> >>
> >> On Mon, Jun 1, 2020 at 7:48 PM Pip Cet <pipcet@gmail.com> wrote:
> >> > > > Indeed, the composition gstring is a single zero-width glyph.
> >> > > See the composition information above: my interpretation of it is that
> >> > > the composed glyph is not zero-width.
> >> >
> >> > ... something is odd here, I agree.
> >>
> >> I think it's a very odd combination of things:
> >> 1. a font which defines an isolated CGJ to have zero width
> >> 2. an isolated CGJ appearing in the first place (in this case, because
> >> another font does not support CGJ)
> >> 3. the fall-back [nil 0 compose-gstring-for-graphic] rule defined for
> >> codepoint #x34f
> >> 4. compose-gstring-for-graphic attempting to salvage non-spacing
> >> characters not following base characters, and producing zero-width
> >> lgstrings from zero-width lglyphs
> >>
> >> Avoiding any of the four will avoid the problem. (1) is something we
> >> cannot fix directly. (2) is also something that a user may want. (3)
> >> could be dropped, and (4) could be expanded to take care of the
> >> zero-width case.
> >>
> >> However, as long as zero-width gstrings can somehow slip through, I
> >> suggest we also apply the patch I sent, assuming it fixes the problem.
> >>
> >> We might consider simply prohibiting zero-width zero-lbearing
> >> zero-rbearing gstrings, the way we prohibit zero-width zero-lbearing
> >> zero-rbearing characters in the code I posted.